Excited-State Energy-Transfer Dynamics in Self-Assembled Triads Composed of Two Porphyrins and an Intervening Bis(dipyrrinato)metal Complex

The synthesis and characterization of various triads composed of a linear array of two zinc porphyrins joined via an intervening bis(dipyrrinato)metal (II) complex are reported. The preparation exploits the facile complexation of dipyrrins with divalent metal ions to give bis(dipyrrinato)metal(II) complexes [abbreviated (dp)2M]. Copper(II) and palladium(II) chelates of dipyrrins (available by oxidation of dipyrromethanes) were prepared in 5080% yield. A one-flask synthesis of bis(dipyrrinato)zinc(II) complexes was developed by oxidation of a dipyrromethane with DDQ or p-chloranil in the presence of Zn(OAc)2·2H2O in THF (~80% yield). Three routes were developed for preparing porphyrin-dipyrrins: (1) Suzuki coupling of a boronatesubstituted zinc porphyrin (ZnP) and bis[5-(4-iodophenyl)dipyrrinato]Pd(II) to give the (ZnP-dp)2Pd triad (50% yield), followed by selective demetalation of the (dp)2Pd unit by treatment with 1,4-dithiothreitol under neutral conditions (71% yield); (2) oxidation of a porphyrin-dipyrromethane with p-chloranil in the presence of Zn(OAc)2·2H2O followed by chromatography on silica gel (71% yield); and (3) condensation of a dipyrrindipyrromethane and a dipyrromethane-dicarbinol under InCl3 catalysis followed by oxidation with DDQ (10-16% yield). Four triads of form (ZnP-dp)2Zn were prepared in 83-97% yield by treatment of a porphyrin-dipyrrin with Zn(OAc)2·2H2O at room temperature. Free base dipyrrins typically absorb at 430-440 nm, while the bis(dipyrrinato)metal complexes absorb at 460-490 nm. The fluorescence spectra/yields and excited-state lifetimes of the (ZnP -dp)2Zn triad in toluene show (1) efficient energy transfer from the bis(dipyrrinato)zinc(II) chromophore to the zinc porphyrins (98.5% yield), and (2) little or no quenching of the resulting excited zinc porphyrin relative to the isolated chromophore. Taken together, these results indicate that bis (dipyrrinato)zinc(II) complexes can serve as self-assembling linkers that further function as secondary light-collection elements in porphyrin-based light-harvesting arrays.